Variable reactors or electronic components of variable reactance, such as variable inductors or variable capacitors are important for tuning purposes (changing frequency) in radio electronics. They may be mechanically or electronically varied, and called upon to handle a variety of power levels from small signals to large signal and power control applications. They may be applied at any frequency where components are available, from audio to the extreme high end of the radio frequency (RF) spectrum.
Inductors are a fundamental electromagnetic component used for a wide variety of purposes in RF circuits. Inductors having large inductances typically include wires wrapped around a bulk dielectric or ferrimagnetic core, and are used in resonant circuits, filters, transformers and other devices. Radio frequency inductors are typically helical coils having an air or ferrite core. Ferrimagnetic core materials are substantially nonconductive and suitable for use at RF and microwave frequencies, while ferromagnetic core materials are generally conductive metallic materials and used at low frequencies.
A variable reactor may involve a biasable media, which may be conductive, dielectric, or magnetic. An example of this is the separation of charge in a semiconductor media, as is accomplished in varicap or varactor diodes. In this device, the media is semiconductor and the bias a DC potential applied thereupon. Varactor diodes are a widespread and inexpensive method for electronic tuning in RF circuits. Yet, their utility may be limited for some tuning requirements, as for instance they may not be suited for high RF power levels due to voltage limitations, and in some instances a variable inductor is needed.
For example, U.S. Pat. No. 6,005,467 to Abramov is directed to a trimmable inductor including a supporting substrate having spaced apart lead terminals, a coil defined by an electrically conductive member mounted on the substrate in a continuous path of multiple turns forming a winding about an axis extending between the lead terminals, and an electric conductive shorting member extending and electrically connected between at least two adjacent windings of the coil to enable selective inclusion and elimination of one of the windings. Cuts are made in the conductors or shorting member to trim the inductor.
Also, U.S. Pat. No. 5,450,052 to Goldberg, et al. is entitled “Magnetically variable inductor for high power audio and radio frequency applications”. The patent discloses a magnetically variable inductor for high power, high frequency applications which includes a solenoid with a magnetic core therein, disposed coaxially around a conductor for carrying the high power, high frequency signal, and a variable current source coupled with the solenoid so that a manipulation of the current through the solenoid results in a variable inductance for the conductor.
Mechanical inductors may include variometers, coils with moveable ferrite slugs, or roller types where a helix is rotated with respect to a “roller” tap. Choices for nonmechanical electrically variable inductors may be more limited. The current state of the art for tunable inductors generally includes, for example: varactor-fixed inductor combinations which may have fine precision, fast speed and are low power handling; relays which are bulky, may have crude precision, medium speed and can offer high power handling; and motor driven types which are bulky, may have fine precision, slow speed and high power handling. However, the combination of fine precision, high speed and high power handling is not available.
There exists a need for an electrically variable, adjustable or tunable inductor of practical size and construction, with high Q/efficiency and having fine precision, high speed and high power handling features. Such a tunable inductor may be used as an antenna pre-selector and/or post-selector filter and is particularly attractive in applications requiring rapid re-tuning. Yet another need would be for antenna loading, as for tuning a small dipole or whip.